This invention relates to electrochromic devices, and particularly to an electrochromic device in which the color center absorption of the electrochromic layer is relatively well matched to the spectral sensitivity of the human eye.
Electrochromic devices are well-known devices which exhibit a phenomenon known as "persistent electrochromism", e.g., see U.S. Pat. No. 3,521,941 entitled, "Electro-Optical Device Having Variable Optical Density," issued July 28, 1970. The term "persistent electrochromism" denotes the property of a material whereby its electromagnetic radiation absorption characteristic is altered, in most instances, even at ambient temperature, under the influence of an electric field. Such materials, for example, may exhibit little or no absorption of visible wavelength in the absence of an electric field, and therefore be transparent, but when subjected to an electric field, effectively absorb in the red end of the spectrum, turning blue in color. Similar effects can be observed in other portions of the electromagnetic spectrum, invisible as well as visible.
Although electrochromic devices have been developed and are successful for many applications, the use of electrochromic materials has been discouraged since many electrochromic materials exhibit color center absorption characteristics which are not well matched to the spectral sensitivity of the human eye. By color center absorption it is meant the optical absorption which arises from impurities or defects added to an otherwise ideal material. This optical absorption can be characterized by an absorption peak and a half-width.
For example, the use of tungsten oxide in electrochromic devices is well known. However, the natural color center absorption of tungsten oxide is relatively broad and peaks at a value which is relatively distant from an ideal peak. The same statement applies also to molybdenum oxide, another well known electrochromic material. If the natural color center absorption peaks of these electrochromic materials were more closely matched to the spectral sensitivity of the human eye, less electrical energy would be required to operate the device at satisfactory viewing levels. In addition to more efficient display operation, the device lifetime would be improved since the device would be operated at lower color center levels. Therefore, it would be desirable to develop an electrochromic display device having an electrochromic layer therein which exhibits color center absorption characteristics which are relatively well matched to the spectral sensitivity of the human eye.